Authors are encouraged to specify a lang attribute on the root html
element, giving the document's language. This aids speech synthesis
tools to determine what pronunciations to use, translation tools to
determine what rules to use, and so forth.

so better, for a page whose content is in American English, would be

<!DOCTYPE html>
<html lang="en-us">

Also if you are using XHTML5 served as application/xhtml+xml you will need to add the namespace, and also the XML equivalent of the lang attribute making it:

According to the HTML5 drafts, “A DOCTYPE is a required preamble”. The preamble <!DOCTYPE html> is recommended, but legacy doctypes are allowed as an alternative, though they “should not be used unless the document is generated from a system that cannot output the shorter string”. The only part that is required in addition to it is the title element, and even it may be omitted under certain conditions. The <html> tag is not required.

HTML5 is not a standard. It is not even a W3C recommendation (yet). What you should use depends on what you are doing. It does not really matter which version of HTML you think you are using. What matters is the markup you have and how browsers (and search engines etc.) process it.